1. Field of the Invention
The present invention relates to a method of pulling up a single crystal used for producing a semiconductor wafer.
2. Description of the Prior Art
FIG. 1 is a side sectional view showing a main section of a single crystal pulling apparatus. In the drawing, 50 is a metallic chamber. The chamber 50 is arranged so that a cylindrical pull chamber 51 is connected with a main chamber 52 of wide-mouthed bottle type. A crucible 31 for melting raw material is positioned in a center of the main chamber 52. The crucible 31 is arranged so that an outer container 33 made of graphite, which has a similar figure to an inner container 32, is fitted to the inner container 32 which is made of quartz and is arranged so that a bowl-shaped lower section is provided to a lower end of a cylindrical drum section. The crucible 31 is rotated and is raised/lowered by a supporting member 35 which is made of graphite and has thermal conductivity approximately same as that of the outer container 33.
Drum-shaped heaters 36 and 37 of resistance heating type are provided on the outside of the crucible 31 so as to be separated from each other in an up-and-down direction and to be on a coaxial circle about the crucible 31. The heights of the heaters 36 and 37 are equal. The heaters 36 and 37 and the crucible 31 are stored in an adiabatic container 40 having a cylindrical shell shape. A hole whose diameter is slightly larger than that of the crucible 31 is provided onto an upper surface of the adiabatic container 40, and a hole whose diameter is slightly larger than that of an axis of the supporting member 35 is provided onto a bottom surface of the adiabatic container 40. The supporting member 35 penetrates the hole provided onto the bottom surface of the adiabatic container 40 and a bottom of the main chamber 52, and the supporting member 35 is connected with a rotary elevator, not shown. Meanwhile, a bar-shaped or wire-shaped pulling shaft 44 is hung from a central axis of the pull chamber 51 so as to be freely raised/lowered and rotated, and a seed crystal 45 is attached to a lower end of the pulling shaft 44.
In order to pull up a single crystal by such an apparatus, the outputs of the heaters 36 and 37 are made to be maximum so as to melt the raw material in the crucible 31, and molten liquid L is obtained. Then, after the outputs of the heaters 36 and 37 are lowered to the same value and the temperature of molten liquid L is lowered to a predetermined value, the seed crystal 45 is brought into contact with the surface of the molten liquid L. While the pulling shaft 44 and supporting member 35 are being rotated respectively in directions opposite to each other, the pulling shaft 44 is pulled up at a predetermined speed, and thus single crystal 46 is grown at a lower position of the seed crystal 45. At this time, as the molten liquid L is reduced by pulling of the single crystal 46, the crucible 31 is raised by the supporting member 35, and the height of the surface of the molten liquid L is maintained approximately constant. When the lower surface of the crucible 31 becomes higher than the upper end of the lower heater 37, the output of the heater 37 is made to be 0.
However, in the conventional single crystal pulling method, the pulled single crystal has a lot of portions where interstitial oxygen concentration is higher than a target value. As for a wafer, which is cut out from a portion where the oxygen concentration in the single crystal is high, at the time of heat treatment, oxygen in the crystal is separated and a defect easily occurs on its surface. Therefore, the wafer cannot be used as a product having required specification, and thus the yields are low. This is, as described in "Single Crystal Growing Apparatus, `Electronic Material, Extra Issue` published in December, 1995", because the oxygen concentration in the single crystal is determined mainly by an amount of oxygen eluted from an inner wall of the bottom of the crucible into the molten liquid, but in the conventional method, the elution of oxygen from the inner wall of the bottom of the crucible cannot be restricted.
In order to solve this problem, the magnetic field applying Czochralski method has been developed. In this method, a magnetic field applying apparatus for applying a magnetic field is provided on the outside of the main chamber, and while a magnetic field is being applied to molten liquid in a crucible, single crystal is pulled up. However, when using the magnetic field applying Czochralski method, since the magnetic field applying apparatus should be installed, the cost of the apparatus is high and an outer dimension of the pulling apparatus is large. For this reason, there arises a problem such that a floor space of a clean room for storing the pulling apparatus becomes large or the number of the pulling apparatuses to be installed in the clean room is decreased.